Michael Moore stands in front of Trump Tower with provocative sign

Liberal activist Michael Moore arrived at Donald Trump's gilded Manhattan skyscraper Wednesday equipped with a homemade sign and a cryptic message for the Republican presidential front-runner: "We are all Muslim."

The 61-year-old filmmaker, known for his Oscar-winning documentaries and anti-capitalist rants, later took to Facebook to describe, in a scathing letter to Trump, why he's taking him on.

"Here we are in 2015 and, like many other angry white guys, you are frightened by a bogeyman who is out to get you. That bogeyman, in your mind, are all Muslims. Not just the ones who have killed, but ALL MUSLIMS," Moore wrote, after recalling the first time he and Trump met while appearing on a talk show together in 1998.

According to Moore, Trump's controversial proposal to bar all non-American Muslims from entering the U.S. was fueled by "desperation and insanity."